1. Field of the Invention
This invention relates to a power transmitting system of a four-wheel drive vehicle, in which front and rear wheels can be driven by a common engine.
2. Description of the Prior Art
Such power transmitting systems of four-wheel drive vehicles are known, and one example thereof is that a front wheel axle is directly connected to an engine, a rear wheel axle is connected through a hydraulic clutch to the front wheel axle, and the drive connection of front wheels to rear wheels can be achieved by applying, to the hydraulic clutch, a hydraulic pressure produced by a hydraulic pump in response to the relative rotation of both the axles.
A gear pump or vane pump generally used as a hydraulic pump for such a power transmitting system is accompanied by a problem that the direction of oil discharged from the pump is determined by the direction of rotation of the pump and hence, the direction of oil discharged is reversed between during forward and backward movements of the vehicle. It is a conventional practice to compensate for the reversal of the direction of oil discharged from the hydraulic pump between forward and backward movements of the vehicle by manual or automatic switchover of a selector or switchover valve provided in an oil passage which connects the hydraulic pump with a working hydraulic pressure chamber in the hydraulic clutch (for example, see Japanese Patent Application No. 238244/90).
With a construction in which the switchover valve is manually switched over between the forward and backward movements of the vehicle in the above manner, however, it is necessary to provide a mechanism for operating the switchover valve. In addition, with a construction in which the switchover valve is automatically switched over, the structure of the switchover valve becomes complicated, bringing about the possibility that trouble such as a locking of a spool may be generated.